DT-1200ADVANCED MECHANICAL DRAFTINGLECTURE #11

POSITION Controls locations of mating featuresIDEAL CONDITIONACCEPTABLE CONDITIONUNACCEPTABLE CONDITION

POSITION ZONE TYPESZones are located from datums with basic dimensionsZone sizes are specified in feature control framesZone is round () when it locates an axis (hole, post,)Zone is flat (no ) when it locates a center plane (slot, tab)Zone size may be modified (,) when dependent on feature size

POSITION TOLERANCE ZONE FOR HOLESHOLE ATTRUE POSITIONACCEPTABLE LOCATIONAL VARIATIONACCEPTABLE ANGULAR VARIATIONIDEAL HOLE LOCATION = CENTER OF ZONEACTUAL HOLE AXIS MUST LIE INSIDE ZONE

FIXED and FLOATING fastenersONE CLEARANCE HOLETWO CLEARANCE HOLESFIXEDFLOATING

CALCULATING POSITION TOLERANCE WHEN USING FASTENERSFLOATING FASTENER CONDITIONTOLERANCE = MMC CLR HOLE MMC FASTENER MMC = largest screw & smallest clearance holeFIXED FASTENER CONDITION

DETERMINING MMC SIZES OF HOLES & FASTENERS FOR CALCULATIONSRULE #1The MMC of a fastener is its nominal sizeFact #1 - Fastener companies manufacture screws so that their NOMINAL SIZES are at MMC (worst case is nominal size)RULE #2The MMC of a drilled hole is its nominal sizeFact #2 Drill bits create holes that are always larger than their NOMINAL SIZES (worst case is nominal size for calculations)

The MMC modifier The MMC modifier provides bonus tolerance whenever the tolerance zone size is based on the feature sizeAllowable movement of hole with hole at MMCEnlarged hole provides for more movement of holeZONE SIZE CAN INCREASE

MMC questionIf feature size deviates from MMC, can the feature move farther out of position and still function correctly?If answer is yes, belongs in feature control frame

Example call out hole for 3/8 screw in fixed conditionNominal size of 3/8 screw is 3/8 (.375)Clearance hole for 3/8 screw is made with drill bit X (.3970)TOLERANCE ZONE DIAMETER = (.397 - .375) 2 = .011Establish datumsLocate tolerance zoneDefine hole and zoneON THE DRAWING

Calculating Actual Zone Size in InspectionMMC of hole = .497Actual Zone Size = MMC zone size + feature size deviation from MMC

The MMC modifier is a powerful tool for allowing bonus tolerance in the location of a feature without decreasing its function or qualityBonus tolerance= faster manufacturing time= less manufacturing cost= MORE MONEY FOR YOU

YOKE FORGINGPhantom lines indicate areas where extra material will be machined offDimensions reflect finished features

Yoke Holes